The present invention relates to a control rod for nuclear reactors and which has elongated channels containing boron carbide or other neutron-absorbing material that swells upon neutron irradiation.
A known control rod of the mentioned kind includes a number of absorber plates in which powdered boron carbide is arranged in elongated, horizontal channels in the form of holes bored in the plates.
Other known control rods include, among other things, absorber plates in which powdered boron carbide is arranged in elongated, vertical channels in the form of empty spaces in tubes arranged adjacent to each other in the plates.
In known finger-type control rods, boron carbide is arranged in elongated, vertical channels in the form of empty spaces in tubes arranged in spaced relationship to each other.
When boron carbide is subjected to neutron irradiation, it absorbs neutrons and forms helium and lithium. This process causes the boron carbide to undergo swelling, which in turn may cause stress corrosion in the construction material around the channel where the boron carbide is located, and hence a risk of cracking. The risk of cracks in the construction material caused by stress corrosion increases with increased irradiation and increased swelling of the boron carbide. It is therefore important to take measures which counteract harmful consequences of the swelling.